Staphylococcus aureus is a major human pathogen causing significant morbidity and mortality in both community- and hospital-acquired infections. Concern over the emergence of multidrug resistant strains, particularly strains which lack sensitivity to all currently available antibiotics, has renewed interest in understanding the virulence mechanisms of this pathogen at the molecular level and in elucidating host defense elements which either provide protection from or which limit infection. Neutrophils (PMN) have long been thought to provide significant host defense against S. aureus infection. However, our studies of S. aureus-induced peritonitis and sepsis in mice have suggested that PMN have both a protective and a deleterious role. In order to demonstrate that PMN contribute to the pathogenesis of S. aureus infection, we have used multiple approaches which either limit or promote PMN migration into the infectious site. Our data indicate that excessive numbers of PMN and elevated levels of a C-X-C chemokine, MIP-2, at the site of a S. aureus infection create an environment which leads to enhanced extracellular replication of the pathogen and its intracellular survival in PMN to the detriment of the host; that PMN isolated from this environment are sufficient to establish infection in naive animals; that some of the bacteria inside these infected PMN are in endosomes with partially or fully degraded membranes; and that two regulatory loci mutants (agr- and sar-) which lack the expression of several virulence factors are less able to survive and/or avoid clearance in the presence of excess PMN and MIP-2. We hypothesize that S. aureus manifests as a virulence determinant the ability to exploit the host's inflammatory response in order to enhance its survival. Moreover, we hypothesize that exogenous modulation of the inflammatory response is sufficient to alter the susceptibility of the host to infection. To test this hypothesis, we will pursue the following specific aims: #1) determine the number of PMN necessary for protection and for their deleterious role in two models of S. aureus infection; #2) define the contribution of C-X-C chemokines, the CXCR2 receptor, and specific virulence factors expressed by S. aureus to the creation of the environment which leads to both enhanced extracellular replication and intracellular survival of the pathogen; #3) elucidate known virulence factors whose genes are activated both in vivo and in vitro specifically in the presence of C-X-C chemokines and PMN; and #4) determine the mechanism of uptake and the intracellular locale of wild-type and isogenic mutants of S. aureus taken up both in vivo and in vitro by C-X-C chemokine-stimulated PMN.